1. Field of the Invention
The invention relates to an abnormality detection apparatus for an exhaust gas control apparatus.
2. Description of the Related Art
For example, as an exhaust gas control apparatus, a Diesel Particulate Filter (DPF) that traps particulate matter (PM) contained in exhaust gas of a diesel engine is known, and an abnormality detection apparatus that detects an abnormality of a DPF is known. For example, an apparatus is known which detects an abnormality of a DPF based on the pressure difference across the DPF (for example, refer to Japanese Patent Application Publication No, 2009-215926).
The abnormality detection apparatus described above, however, may fail to perform the abnormality detection properly when the concentration of PM in exhaust gas is low and thus the change in the exhaust gas pressure difference across the DPF is small. Meanwhile, a method may be used in which whether a DPF is in an abnormal state is determined by directly measuring PM using a sensor. In this method, however, the output value of the sensor is small when the concentration of PM in exhaust gas is low, and therefore, in such a case, the abnormality detection may not be properly performed.